1. Field of the Invention
This invention relates generally to food packaging films, more particular to films in which packaged food products can be cooked.
2. Background of the Invention
Many food products are processed in thermoplastic film packages by subjecting the packaged product to elevated temperatures produced by, for example, immersion in hot water or exposure to steam. Such thermal processing often is referred to as cook-in, and films used in such processes are known as cook-in films.
A food product that is packaged and processed in this manner can be refrigerated, shipped, and stored until the food product is to be consumed or, for example, sliced and repackaged into smaller portions for retail display.
Many sliced luncheon meats are processed in this fashion. Alternatively, the processed food can be removed immediately from the cook-in package for consumption or further processing (e.g., sliced and repackaged).
A cook-in film must be capable of withstanding exposure to rather severe temperature conditions for extended periods of time while not compromising its ability to contain the food product. Cook-in processes typically involve a long cook cycle. Submersion in hot (i.e., about 55xc2x0 to 65xc2x0 C.) water for up to about 4 hours at is common; submersion in 70xc2x0 to 100xc2x0 C. water or exposure to steam for up to 12 hours is not uncommon, although most cook-in procedures normally do not involve temperatures in excess of about 90xc2x0 C. During such extended periods of time at elevated temperatures, any seams in a package formed from a cook-in film preferably resist failure (i.e., pulling apart).
Following the cook-in process, the film or package preferably conforms, if not completely then at least substantially, to the shape of the contained food product. Often, this is achieved by allowing the film to heat shrink under cook-in conditions so as to form a tightly fitting package. In other words, the cook-in film desirably possesses sufficient shrink energy such that the amount of thermal energy used to cook the food product also is adequate to shrink the packaging film snugly around the contained product. Alternatively, the cook-in film package can be caused to shrink around the contained food product prior to initiating the cook-in procedure by, for example, placing the package in a heated environment prior to cooking.
The cook-in film also preferably possesses sufficient adherence to the food product to inhibit or prevent xe2x80x9ccook-outxe2x80x9d (sometimes referred to as xe2x80x9cpurgexe2x80x9d), which is water and/or juices that collect between the surface of the contained food product and the food-contact surface of the packaging material during the cook-in process. Preventing cook-out can increase product yield, provide a better tasting product, improve shelf life and provide a more aesthetically appealing packaged product. Films that adhere well to the packaged food product help reduce cook-out.
Many cook-in films are corona treated to increase the surface energy of their food-contact layers. However, corona treatment can be inconsistent, can result in a film with inconsistent adhesion, can result in a film having a surface energy that decays over time, and can interfere with the sealability of a film.
Many types of meat are processed by a cook-in procedure. Common examples include ham, sausage, some types of poultry, mortadella, bologna, braunschweiger, and the like. However, such meats can vary substantially in fat and protein content. Obtaining adequate film-to-meat adhesion becomes more difficult with respect to meats that are high in fat, low in protein, or have substantial levels of additives (starch, water, etc.). Adhesion of the film to the meat product is believed to be due to polar functionalities of the protein being attracted to polar functionalities on the surface of the cook-in film. For example, poultry has a relatively low fat content and a relatively high protein content; therefore, obtaining adequate film-to-poultry meat adhesion is relatively easy. However, ham, sausage, mortadella, bologna, braunschweiger and the like have relatively high fat contents and relatively low protein content; therefore, obtaining adequate film-to-meat adhesion for such meat products (especially sausage, mortadella, bologna, and braunschweiger) is more difficult.
Some presently available cook-in films provide excellent adhesion with the meat product and do a good job of reducing cook-out. Additionally, most presently used films are able to withstand extended time periods at the elevated temperatures described supra; accordingly such films are adequate for many cook-in applications. However, some cook-in applications impose even more stringent performance requirements. For example, some food products that are processed via cook-in procedures are oxygen sensitive. Cook-in films for these products need to include one or more oxygen barrier layers. Other cook-in applications require that the film or the package made therefrom be printable and be able to retain any image printed thereon.
One of the most troublesome of these performance requirements is durability when used in conjunction with a forming shoe (during the package forming process). Where a film has a high degree of shrinkability in the transverse direction, it tends to xe2x80x9cneck downxe2x80x9d on the forming shoe during the sealing step of the process. This often causes such highly shrinkable films to rupture.
No presently available cook-in film is believed to possess all of the following characteristics: (1) good adherence to protein, (2) an extremely low permeance to oxygen, (3) an ability to shrink around a packaged product in a controlled fashion, (4) a seal layer with a softening point that is sufficiently high to survive cook-in conditions, (5) an ability to be sealed around a forming shoe without necking down, (6) good resistance to clip cuts, and (7) an ability to be printed in such a manner that the printed image is protected during the cook-in process as well as in subsequent transport and handling.
Briefly, the present invention provides a multilayer structure that includes a first polymeric film laminated to a second polymeric film. At least one of the films includes a barrier layer with an oxygen permeance of no more than about 150 cm3/m2xc2x7atmxc2x724 hours at about 23xc2x0 C. and 0% relative humidity. (The units for oxygen permeance as used herein throughout are fairly common in the industry. To convert these to SI units, mol/m2xc2x7sxc2x7Pa, one need only multiply by a factor of 5.097xc3x9710xe2x88x9215.) Each of the polymeric films include an outer layer that forms an outer surface of the multilayer structure (i.e., an outer layer of the first polymeric film forms one outer surface of the multilayer structure while an outer layer of the second polymeric film forms the other outer surface of the multilayer structure). The aforementioned outer layer of the first polymeric film, even when untreated, has a surface energy of at least 0.034 J/m2 and includes at least about 10% (by wt.) of a polymer having a Vicat softening point of at least about 65xc2x0 C. The outer surface of the multilayer structure formed from the outer layer of the first polymeric film (i.e., the first outer surface) can be sealed to itself, the opposite outer surface (i.e., the second outer surface), or an optional adhesive tape applied over a butt-seam juncture formed by contacting the first outer surface with itself. Each of the foregoing sealing techniques can result in the formation of a tube which, through further sealing and cutting techniques well known to those of ordinary skill in the art, can result in packages.
In another aspect, the present invention provides a multilayer structure that includes a first polymeric film laminated to a second polymeric film, at least one of which includes a barrier layer with an oxygen transmission coefficient of no more than about 150 cm3/m2xc2x7atmxc2x724 hours at about 23xc2x0 C. and 0% relative humidity. Each of the polymeric films includes an outer layer that forms an outer surface of the multilayer structure. The aforementioned outer layer of the first polymeric film includes at least one of (1) a polymer that includes mer units derived from a C2-C4 xcex1-olefin and at least 2 weight percent mer units derived from a C3-C18 unsaturated acid, (2) an anhydride-modified polymer that includes mer units derived from a C2-C4 xcex1-olefin, (3) a polymer that includes mer units derived from lactic acid, (4) a polyamide, (5) a polyester, and (6) a polyurethane. The sealing characteristics and capabilities of the outer surfaces of this multilayer structure are the same as those set forth for the multilayer structure defined in the previous paragraph.
Both of the multilayer structures just described are believed to possess each of the six characteristics set forth in the previous section of this document. As such, they are ideal for use as films for use in many, if not all, cook-in applications.
To assist in understanding the more detailed description of the invention that follows, certain definitions are provided immediately below. These definitions apply herein throughout unless a contrary intention is explicitly indicated:
xe2x80x9cpolymerxe2x80x9d means the polymerization product of one or more monomers and is inclusive of homopolymers as well as copolymers, terpolymers, tetrapolymers, etc., and blends and modifications of any of the foregoing;
xe2x80x9cmer unitxe2x80x9d means that portion of a polymer derived from a single reactant molecule; for example, a mer unit from ethylene has the general formula xe2x80x94CH2CH2xe2x80x94;
xe2x80x9chomopolymerxe2x80x9d means a polymer consisting essentially of a single type of repeating mer unit;
xe2x80x9ccopolymerxe2x80x9d means a polymer that includes mer units derived from two reactants (normally monomers) and is inclusive of random, block, segmented, graft, etc., copolymers;
xe2x80x9cinterpolymerxe2x80x9d means a polymer that includes mer units derived from at least two reactants (normally monomers) and is inclusive of copolymers, terpolymers, tetrapolymers, and the like;
xe2x80x9cpolyolefinxe2x80x9d means a polymer in which some mer units are derived from an olefinic monomer which can be linear, branched, cyclic, aliphatic, aromatic, substituted, or unsubstituted (e.g., olefin homopolymers, interpolymers of two or more olefins, copolymers of an olefin and a non-olefinic comonomer such as a vinyl monomer, and the like);
xe2x80x9c(meth)acrylic acidxe2x80x9d means acrylic acid and/or methacrylic acid;
xe2x80x9c(meth)acrylatexe2x80x9d means acrylate and/or methacrylate;
xe2x80x9canhydride functionalityxe2x80x9d means any group containing an anhydride moiety, such as that derived from maleic acid, fumaric acid, etc., whether blended with one or more polymers, grafted onto a polymer, or polymerized with one or more monomers;
xe2x80x9coxygen permeancexe2x80x9d (in the packaging industry, xe2x80x9cpermeancexe2x80x9d often is referred to as xe2x80x9ctransmission ratexe2x80x9d) means the volume of oxygen (O2) that passes through a given cross section of film (or layer of a film) at a particular temperature and relative humidity when measured according to a standard test such as, for example, ASTM D 1434 or D 3985;
xe2x80x9clongitudinal directionxe2x80x9d means that direction along the length of a film, i.e., in the direction of the film as it is formed during extrusion and/or coating;
xe2x80x9ctransverse directionxe2x80x9d means that direction across the film and perpendicular to the machine direction;
xe2x80x9cfree shrinkxe2x80x9d means the percent dimensional change, as measured by ASTM D 2732, in a 10 cmxc3x9710 cm specimen of film when subjected to heat;
as a verb, xe2x80x9claminatexe2x80x9d means to affix or adhere (by means of, for example, adhesive bonding, pressure bonding, corona lamination, and the like) two or more separately made film articles to one another so as to form a multilayer structure; as a noun, xe2x80x9claminatexe2x80x9d means a product produced by the affixing or adhering just described;
xe2x80x9cdirectly adhered,xe2x80x9d as applied to film layers, means adhesion of the subject film layer to the object film layer, without a tie layer, adhesive, or other layer therebetween;
xe2x80x9cbetween,xe2x80x9d as applied to film layers, means that the subject layer is disposed in the midst of two object layers, regardless of whether the subject layer is directly adhered to the object layers or whether the subject layer is separated from the object layers by one or more additional layers;
xe2x80x9cinner layerxe2x80x9d or xe2x80x9cinternal layerxe2x80x9d means a layer of a film having each of its principal surfaces directly adhered to one other layer of the film;
xe2x80x9couter layerxe2x80x9d means a layer of a film having less than both of its principal surfaces directly adhered to other layers of the film;
xe2x80x9cinside layerxe2x80x9d means the outer layer of a film in which a product is packaged that is closest, relative to the other layers of the film, to the packaged product;
xe2x80x9coutside layerxe2x80x9d means the outer layer of a film in which a product is packaged that is farthest, relative to the other layers of the film, from the packaged product;
xe2x80x9cbarrier layerxe2x80x9d means a film layer capable of excluding one or more gases (e.g., O2);
xe2x80x9cabuse layerxe2x80x9d means an outer layer and/or an inner layer that resists abrasion, puncture, and other potential causes of reduction of package integrity and/or appearance quality;
xe2x80x9ctie layerxe2x80x9d means an inner layer having the primary purpose of providing interlayer adhesion to adjacent layers that include otherwise nonadhering polymers;
xe2x80x9cbulk layerxe2x80x9d means any layer which has the purpose of increasing the abuse resistance, toughness, modulus, etc., of a multilayer film and generally comprises polymers that are inexpensive relative to other polymers in the film which provide some specific purpose unrelated to abuse resistance, modulus, etc.;
xe2x80x9cseal layerxe2x80x9d (or xe2x80x9csealing layerxe2x80x9d or xe2x80x9cheat seal layerxe2x80x9d or xe2x80x9csealant layerxe2x80x9d) means;
(a) with respect to lap-type seals, one or more outer film layer(s) (in general, up to the outer 75 xcexcm of a film can be involved in the sealing of the film to itself or another layer) involved in the sealing of the film to itself, another film layer of the same or another film, and/or another article which is not a film, or
(b) with respect to fin-type seals, an inside film layer of a package, as well as supporting layers within 75 xcexcm of the inside surface of the innermost layer, involved in the sealing of the film to itself;
as a noun, xe2x80x9csealxe2x80x9d means a bond of a first region of a film surface to a second region of a film surface (or opposing film surfaces) created by heating (e.g., by means of a heated bar, hot air, infrared radiation, ultrasonic sealing, etc.) the regions (or surfaces) to at least their respective softening points;
xe2x80x9cclip cutxe2x80x9d means a reduction in package integrity due to either or both of the film gathering device and the application of a wire or preformed clip used to seal an end of the package; and
xe2x80x9ccookxe2x80x9d means to heat a food product thereby effecting a change in one or more of the physical or chemical properties thereof (e.g., color, texture, taste, and the like).
Films used in the food packaging industry often are categorized according to the number of layers that make up the film. Some films are made from a single polymer or blend of polymers and thus have only one layer. However, most films include more than one layer and are referred to as multilayer films. In general, the layers of a multilayer film can be classified as inner or outer layers. Normally, inner layers are included to provide additional or different properties to the film. In addition, any number of tie layers can be included in the film. Such tie layers can be present primarily on the outside of an inner layer or can be a layer itself. Many tie layers include one or more modified polyolefins and/or polyurethanes, more preferably modified ethylene/xcex1-olefin copolymer, modified ethylene/unsaturated ester copolymer, and/or modified ethylene/unsaturated acid copolymer. Anhydride-modified ethylene/xcex1-olefin copolymer and anhydride-modified ethylene/unsaturated ester copolymer are particularly preferred. Specific examples include anhydride-grafted linear low density polyethylene and anhydride-grafted ethylene/vinyl acetate copolymer.
With respect to films used for cook-in processes in general, one outer layer acts as a food-contact layer while the other acts as an outside layer. The former serves as the inner layer of a package formed from the film and is in direct contact with the packaged food product. The latter provides abuse resistance by serving as the outermost layer of the package.
Some films, including many which are used in cook-in processes, are oriented prior to use. Orientation involves stretching a film at an elevated temperature (the orientation temperature) followed by setting the film in the stretched configuration (e.g., by cooling). When an unrestrained, unannealed, oriented polymeric film subsequently is heated to its orientation temperature, heat shrinkage occurs and the film returns almost to its original, i.e., pre-oriented, dimensions.
An oriented film has an orientation ratio, which is the multiplication product of the extent to which the film has been expanded in several directions, usually two directions perpendicular to one another. Expansion in the longitudinal direction, sometimes referred to as the machine direction, occurs in the direction the film is formed during extrusion and/or coating. Expansion in the transverse direction means expansion across the width of the film and is perpendicular to the longitudinal direction.